In a conventional electrostatographic copying process, a laten electrostatic image is formed on an insulating substrate, such as a photoconductor. If a dry development process is used, charged toner particles are applied to the electrostatic image, where they adhere in proportion to the magnitude of the electrostatic potential difference between the toner particles and the charges on the image. Toner particles that form the developed image are transferred to a receiver by pressing the surface of the receiver against the developed image. It is conventional to use either an electrostatically biased roller or a corona to transfer toner particles from the image bearing substrate to the receiver. The transferred particles are then fixed to the receiver surface by a suitable method such as the application of heat.
While this conventional process works well with large toner particles, difficulties arise as the size of the toner particles is reduced. Smaller toner particles are necessary to achieve higher resolution copies but, as the size of the toner particles falls below about 8 micrometers, the surface forces holding the toner particles to the substrate tend to dominate over the electrostatic force that can be applied to the particles to assist their transfer to the receiver. Thus, less toner transfers and image quality suffers increases in mottle. In addition, as the particle size decreases, certain other image defects also begin to increase, such as the "halo defect," where tone particles that are adjacent to areas of maximum toner density fail to transfer, and "hollow character," where the centers of fine lines fail to transfer. "Dot explosion," where toner particles comprising half tone dots scatter during transfer, also occurs during electrostatic transfer. Some of these defects are believed to be due to repulsive coulombic forces between the particles. This, high resolution images require very small particles, but high resolution images without image defects have not been achievable using electrostatically assisted transfer.
One alternative process of transferring toner particles, without using an electrostatic bias, is to melt or fuse the particles to the receiver during transfer by heating the toner above its melting point. While this process does ameliorate image quality by reducing the defects that are aggravated by electrostatically assisted transfer, it, in turn, creates new problems that must be overcome. First, that process requires higher temperatures than does the conventional process, and these higher temperatures subject the substrate (e.g., a photoconductor) to higher temperatures. This can alter the electrical and photoconductive characteristics of the substrate, and/or cause physical distortions, and therefore mandate the use of more thermally stable materials, which may be more expensive and/or less suitable for other reasons. The receiver is also subjected to higher temperatures over a long period of time which can weaken and deteriorate the receiver and blister its surface. Also, because of the time required for enough heat to transfer from the receiver to the toner to melt it, the process is slow; typical process speeds are of the order of only 0.4 meters/minute. Melted toner may also occasionally fuse to the substrate, which may permanently damage the substrate. A special cleaning process is also needed if the substrate is to be reused, and cleaning adds to the cost of the process and subjects the substrate to additional thermal cycling. High pressures (about 345 to 760 kPa) are also needed in this process. These high pressures, in conjunction with the high temperature and long nip duration time, can be especially hard on a substrate.